In the fracturing of subterranean formations for the production of hydrocarbons, it is common to produce fractures in the formation by pumping a fluid at a relatively high pressure into the formation through a wellbore whereby the formation fracturing pressure gradient is exceeded and fractures propagate from the wellbore into the formation. Nearly any fluid given enough volume and pressure can be used to fracture a subterranean formation. However, fracturing fluids, generally include a viscosifying or gelling agent such as a crosslinked or un-crosslinked polysaccharide material, or a viscoelastic surfactant, to affect the rheology by increasing viscosity and increase the proppant-carrying capability of the fluid.
Using stimulation operations such as fracturing, gravel packing, frac-packing, and combinations thereof in high-permeability formations, that is formations having a permeability greater than 100 millidarcy (>100 md; >10−9 cm2) has generated considerable interest in recent years. Attempts have been made to utilize hydroxyethylcellulose (HEC) as a viscosifier in the fracturing fluids for such formations. Unfortunately, HEC and most other polymer gels exhibit high rates of leak off to the formation because they may not build up a satisfactory filter cake with acceptable volumes of lost fluid at permeabilities >100 md (>10−9 cm2). This results in large volumes of fluid being required for each formation treatment. The high fluid loss also makes it very difficult to create a fracture having the desired geometry to maximize hydrocarbon production because the fluid is leaking off into the pores instead of pushing against the rock.
Methods are known for fracturing relatively high permeability subterranean formations (above about 10 md (above about 10−10 cm2) using a fracturing fluid system employing two polymer-gelled fluids. An aqueous fracturing fluid is prepared including a viscosifying agent, such as a borate crosslinked hydroxypropyl guar gel. The crosslinked gel is introduced into a subterranean formation through a wellbore at a rate and pressure sufficient to result in initiation of a fracture in the formation and development of a filter cake to control fluid-loss. The first fluid also may include a quantity of proppant. A second fracturing fluid also is prepared. The second fluid comprises a viscosifying agent and may or may not include a crosslinking agent for the viscosifying agent; preferably not. A proppant is added to the second gel and the fluid is introduced into the formation and into the fracture created by the first fluid. The second fluid functions to carry and transport the proppant into the created fracture and, more importantly, to induce a break of the filter cake formed from the first fluid. The second fluid causes an increase in the leak-off rate of the fluid through the fracture faces which improves the ability of the proppant to pack within the fracture by dehydration of the fracturing fluid. Unfortunately, the use of even one polysaccharide-containing fluid tends to cause formation damage when the filter cake formed is removed. Using two polysaccharide-containing fluid increases this risk.
It would be desirable to provide a method by which a high permeability formation could be successfully stimulated while minimizing the potential for formation and proppant bed damage. This method should still reduce or eliminate the tendency of the fluid to leak off into the high permeability formation to make the overall treatment method more efficient and less wasteful.